The present invention relates to data communications, and more particularly, to methods and arrangements for recovering digitally modulated data in a differential phase shift keyed signal.
Differential phase shift keying (DPSK) is a digital modulation technique that is used to encode information in a signal prior to transmitting the signal to one or more receivers. The information in a DPSK modulated signal is carried by the transition between distinct digital states rather than the absolute digital state, hence the term differential. There are several DPSK modulation techniques, including, for example, differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK).
In DBPSK modulation, the phase of a constant amplitude carrier signal shifts between two positions, for example, 0 degrees and 180 degrees. As such, two distinct digital states are provided, for example, a binary xe2x80x9c0xe2x80x9d at 0 degrees and a binary xe2x80x9c1xe2x80x9d state at 180 degrees.
In DQPSK modulation, the phase of a constant amplitude carrier signal shifts between four positions, typically in increments of 90 degrees (e.g., 45, 135, xe2x88x92135 and xe2x88x9245 degrees). As such, DQPSK signals typically provide four distinct digital states, for example, a binary xe2x80x9c00xe2x80x9d at 45 degrees, a binary xe2x80x9c01xe2x80x9d at 135 degrees, a binary xe2x80x9c10xe2x80x9d at 225 degrees (or xe2x88x92135 degrees), and a binary xe2x80x9c11xe2x80x9d at 315 degrees (or xe2x88x9245 degrees).
In both DBPSK and DQPSK, information is differentially encoded in the transition from one state to the next, and/or a lack of thereof. As such, there is no need to recover or track the carrier signal within the receiver, as there would be in a receiver that is configured to demodulate conventional phase shift keyed signals. Instead, the receiver only needs to identify the relative phase shift between two adjacent bits to identify the information contained in a differentially encoded signal. This tends to simplify the design of the receiver. For example, a single channel receiver can be used to receive and demodulate a DBPSK signal.
There are, however, problems with single channel receivers. For example, a single channel receiver needs to provide an accurate delay time between bits in order to properly detect the phase shift and demodulate the signal. Further, small differences or jitters in the delay time tend to increase the resulting bit error rate (BER).
To avoid such problems quadrature demodulation of DBPSK signals can be employed. By applying quadrature demodulation techniques to a DBPSK signal, phase vectors are produced, which form a constellation when mapped on to a complex plane. The location of the resulting data points in the constellation can then be used to identify the xe2x80x9ctruexe2x80x9d value of the originally encoded data.
Identifying the xe2x80x9ctruexe2x80x9d value of the originally encoded data is further complicated by changes that occur to the DBPSK signal during transmission. For example, the medium through which the signal is transmitted typically alters the signal by xe2x80x9ctwistingxe2x80x9d or otherwise changing the phase if either the sine and/or cosine signals within the transmitted signal. As such, the received phase of one or more of the components in the received signal may be different from the transmitted phase.
To correct for unwanted phase changes, the sender and receiver will typically attempt to characterize the affects of the medium during an initial transmission sequence. For example, the sender may send a series of differentially encoded binary 1""s to the receiver for a specified duration. The receiver will then attempt to recognize the series and possibly identify the characteristics of the medium. A conventional receiver accomplishes this by sampling the received signal and applying certain decisions, for example, using logic circuitry, to increase the probability of correctly determining the original data sent based on the constellation. Such decisions can be represented in a decision or constellation table.
Since there are many dynamic and static factors involved in a transmission, different constellation tables (and logic) have been created in an effort to provide reduced BERs. For example, in certain receivers advanced logic is combined with highly accurate analog-to-digital converters to sample the received analog sine and/or cosine signals and determine the vector intercepts in the constellation. While such complex receivers tend to have lower BERs, they can be very costly.
Therefore, as can be appreciated, there is a continuing need for improved methods and apparatus for use in receiving DPSK signals. Preferably, the methods and apparatus will be simple and cost effective to implement, while also providing acceptably low BERs.
In accordance with certain aspects of the present invention, methods and apparatus are provided that allow differential encoded phase shift keyed signals to be received, demodulated and decoded using simple, cost effective sampling, decoding and decision techniques that result in a reasonably low bit error rate (BER).
For example, in accordance with one aspect of the present invention, an analog signal that is digitally modulated using differential binary phase shift keying (DBPSK) and transmitted over a communication medium, is demodulated by a quadrature demodulator into two signals that are then converted into two digital signals using 2-bit analog-to-digital converters. Samples of these digital signals are then encoded according to a maximum likelihood criteria algorithm and decisions are made based on the signal constellation, which identifies the original data transmitted.
In accordance with further aspects of the present invention, a method is provided for accurately identifying the original data within a differential phase shift keyed (DPSK) signal. The method includes the steps of taking a first digital sample of at least a portion of the DPSK signal at a first time, and taking a second digital sample of the at least a portion of the DPSK signal at a subsequent second time. Then the method includes producing a coded digital sample from the first and second digital samples. In accordance with certain further embodiments of the present invention, the first and second digital samples are 2-bit digital samples resulting from an analog-to-digital conversion of portions of the DPSK signal at different, closely-spaced, times, such as, for example, 100 nanoseconds apart. In accordance with still other embodiments of the present invention, the coded digital sample is produced by combining the 2-bit first digital sample with the 2-bit second digital sample to produce a 4-bit coded digital sample. In accordance with still further embodiments of the present invention, the apparatus also includes a receiver that is configured to receive a transmitted DPSK signal and output at least a portion of the transmitted DPSK signal to the sampler as the DPSK signal.